1. Field of the Invention
The present invention relates to a time division multiple access (TDMA) communication system, and more particularly to a method for forming a frame structure which is suitable for effectively transmitting voice and data.
2. Description of the Related Art
With the development of the modern information society, the demand for mobile telecommunications is increasing rapidly. In particular, arise demands not only for the voice communication, but also for the non-voice communication services such as data, facsimile, and video communication based on ISDN (Integrated Services Digital Network). In general, in order to increase the capacity for accommodating subscribers, the mobile telecommunication employs a digital cellular mobile telecommunication system, which has a high frequency recycling efficiency and provides an effective non-voice service. The digital cellular mobile telecommunication system has different a system capacity, operating methods, and facility of realizing the system, according to the multiple access technique.
The multiple access allows a plurality of users to share limited radio communication channels. Such multiple access is generally divided into a frequency division multiple access (FDMA), a time division multiple access (TDMA), a code division multiple access (CDMA), and a space division multiple access (SDMA). The digital cellular mobile telecommunication system generally employs TDMA and CDMA. The TDMA system used a common frequency band and allows a plurality of users to share a carrier wave, by using time slots of time intervals (a so-called frame) allocated to each of the users. The time intervals allocated to each of the users should not be overlapped.
Recently, a personal communication network, which is an advanced cellular mobile telecommunication network, has been proposed. In order to process traffics having various kinds of service request conditions, such a personal communication network needs to have a radio access structure for effective bandwidth allocation. In view of the flexibility and the statistical multiplexing gain, it is known that to transmit the voice and data by the packet is most advantageous.
FIG. 1 illustrates a frame structure including the voice and data which are packed into a packet by the prior art TDMA cellular mobile telecommunication system. Such a frame structure is disclosed in a paper entitled "Multimedia Transport in Next-Generation Personal Communication Networks", Proc. ICC'93, 1993, pp. 858-862, by D. Raychaudhuri and N. Wilson, and a paper entitled "Packet CDMA versus Dynamic TDMA for Multiple Access in an Integrated Voice/Data PCN" IEEE J. on Select. Areas Commun., Vol. 11, pp. 870-884, 1993, by N. Wilson, R. Ganesh, K. Joseph and D. Raychaudhuri.
The prior art communication systems using the frame structure as shown in FIG. 1 have the following disadvantages.
(1) Hybrid Switching with Movable Boundary
In this communication system, radio access is achieved based on the frame structure as shown in FIG. 1. As illustrated, the frame is divided into a reservation phase and an allocation phase. In operation, a traffic user reserves a right of using the subsequent slots at the short reservation phase. If the user has made the reservation successfully or data to be transmitted will be allocated at the allocation phase. Here, a voice traffic is transmitted by channel switching, and a data traffic is transmitted by packet switching. Further, the number of slots that each traffic can use is dynamically controlled according to a load of the voice, to guarantee a priority order of the voice, as far as possible.
With use of the foregoing communication system, it is possible to give the priority order to the voice user when allocating the channel resources. However, fundamentally, it is not possible to settle a voice/data contention problem. Therefore, in case the number of the data users increases rapidly, it is not possible to prevent a loss of the voice packets, which may be caused in the course of the contention.
(2) Packet Switching with Different Permission Probabilities
In this communication system, empty slots are reserved according to permission probabilities of the respective voice or data subscribers. If the empty slots are reserved successfully, the reserved slots will be used until the transmission is completed, in case of the voice. Here, it may be possible to increase the opportunities for reserving the voice, by getting the permission probability of a first voice packet to be higher than a data value. However, in case the traffic load of data increases, the quality of the voice may decrease drastically because the voice/data contention problem is coincidently settled.
(3) Voice/Data Contention Separated System
In accordance with this system, a resource that the voice traffic has requested is first allocated, and then the remaining resources are allocated to data terminals, thereby guaranteeing a priority order of the voice. This voice/data contention separated system is divided into an ALOHA-reservation type and an R-ALOHA type according to the settlement procedure of the contentions.
3-1) ALOHA-Reservation Type
In this type, the contention settlement procedure is performed at a reservation mini-slot on a beginning part of the frame, and a voice talkspurt is reserved by the frame unit. Further, the data is transferred at an idle slot remaining in the allocation phase after the voice traffic is reserved. With use of this system, it may be possible to settle the contention problem in some degree. However, when a channel error occurs in the course of settling the contentions, one should wait for a long time of one frame for the next reservation. In such a case, there may be a high probability that the packets will not be used due to a time delay restriction of the voice.
3-2) R-ALOHA Type
In this type, it is possible to reserve the voice at currently unused slots. A data user can use the unused slots by way of the contention procedure thereof, only when voice is not used. In such a case, if the number of the voice traffics increases, most of the slots are used up. Thus, it may not be possible to have an opportunity for reserving the slots to transfer the next packet. Accordingly, in case the voice load is relatively large, a time delay in accessing the voice increases. Further, if the voice fails to use slots due to the contention, the slots are wasted undesirably.